Method for kraft waste reclamation

ABSTRACT

A method for obtaining particulate calcium carbonate having an average particle size less than about 12 microns is provided. The method includes the steps of (1) withdrawing from a pulp mill a mixture containing calcium carbonate; (2) treating the mixture to remove contaminants contained in the mixture to produce a treated mixture containing calcium carbonate and further having a chemical composition and/or purity which substantially inhibits the fusing together of calcium carbonate particulates; (3) recovering from the treated mixture particulate calcium carbonate having an average particle size less than about 12 microns. The calcium carbonate produced has a high surface area to volume ratio and is therefore highly reactive and suitable for numerous applications such as in the treatment of soil, filler paper production, paint production, and contaminant containment in coal stack emission assemblies.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 10/022,086, filedDec. 18, 2001, entitled “KRAFT WASTE RECLAMATION METHODOLOGY;” nowabandoned which is a continuation of U.S. Ser. No. 09/754,754, filedJan. 4, 2001, entitled “KRAFT WASTE RECLAMATION METHODOLOGY,” nowabandoned; which is a continuation of U.S. Ser. No. 09/400,243, filedSep. 21, 1999, entitled “METHOD OF OBTAINING AND USING PARTICULATECALCIUM CARBONATE,” now abandoned; which is a continuation of U.S. Ser.No. 08/861,333, filed May 21, 1997, entitled “METHOD OF OBTAINING ANDUSING PARTICULATE CALCIUM CARBONATE,” now abandoned, the contents of allof which are incorporated herein in their entirety by reference.

FEDERALLY SPONSORED RESEARCH

NOT APPLICABLE

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to a method for obtainingparticulate calcium carbonate and methods for using same, and moreparticularly, to a method for obtaining particulate calcium carbonatehaving an average particle size less than about 12 microns from a pulpmill and further having a chemical composition and/or purity whichsubstantially inhibits the fusing together of the calcium carbonateparticulates.

2. Background of the Art

Limestone, which is primarily calcium carbonate, has been quarried andprocessed for a wide variety of uses pre-dating the construction of thepyramids of Ancient Egypt. The direct use of limestone and theconversion of limestone to quicklime has continued unabated since thattime for use in the construction of buildings and roads, as well as forglass formation and the purification of metals. The advent of theindustrial and technical revolutions has continued the need for highquality calcium carbonate. However, the increased need for calciumcarbonate has brought with it a demand for calcium carbonate particlesizes below that which can be accurately measured by a standard sieveanalysis. Oftentimes, the particulate size requirement is only a fewmicrons in diameter.

The use of calcium carbonate in agricultural settings and manufacturingapplications is also well known in the art. Since World War II,increasing amounts of calcium carbonate materials have been spread onthe soil of farms (hereinafter referred to as “liming”) as a method ofincreasing the productivity of the soil and aligning the soil pH closerto neutral. In fact, the direct application of calcium carbonate tosoil, is the greatest single agricultural use of calcium carbonate. Itsuse in agricultural applications during the 1940-1970 time spanaccounted for approximately 70-80% of the tonnage of calcium carbonateproduced. For example, in Willamette Valley, Oreg., over 150,000 tons oflimestone products are used each year for soil neutralization therebyincreasing the yields of a number of products including grasses forseed.

Traditionally, the calcium carbonate material has been spread byself-unloading dump or tank-type trucks and the calcium carbonate hasbeen applied to the land by plowing about half of the calcium carbonateunder the soil and harrowing in the remaining half. More recently, thesoaring price of fertilizer has made the spreading of particulatecalcium carbonate an attractive and inexpensive option for farmers andother agricultural users. Indeed, it has been found that the preliminarytreatment of agricultural plots with calcium carbonate is a prerequisitein order to reap the full value from such costly fertilizers.

The use of calcium carbonate is varied across a wide spectrum ofapplications. For instance, a preponderance of crops and plants growmost profusely under neutral to slightly acidic conditions. Thus, acidicsoil in the pH range of 3.5-6.0 can be made more fertile and productivefor many crops by neutralizing soil acids. Also, the essential plantnutrients, calcium and magnesium, are supplied directly to the plants tosupport plant growth. Through liming, microbiological activities in thesoil are stimulated, thereby liberating other available plant nutrientsfrom the soil organic matter. Indirectly, liming increases organicmatter in the soil by fostering larger and more prolific growth. Greatervolumes of roots and plant residues are retained in and on the soil andthe earthworm population generally increases as the pH of the soil iselevated up to neutral.

Numerous problems have made the agricultural application of calciumcarbonate incomplete: for instance, calcium carbonate of sufficient sizeand surface area is expensive to obtain and cannot be produced fromlarger sized calcium carbonate economically. Although liming has becomea requisite in the agricultural industry, the application of calciumcarbonate through liming has been partially ineffective, time intensive,and potentially over applied. Furthermore, the application of powderedcalcium carbonate directly to the soil is ineffective, as well, becauseit tends to be blown away by the wind and requires lengthy treatmenttimes to reach the desired soil pH level.

The numerous problems of the agricultural use of calcium carbonate aremirrored and amplified in the use of calcium carbonate for flue gasdesulfurization for the reduction of acid rain, in the power industry.Although low sulfur coals have been utilized in order to reduce sulfurdioxide produced, thereby postponing the installation of expensivescrubbers, tightening environmental regulations will soon force powerplants to also use flue gas desulfurization “scrubbing” techniques.However, the size of the calcium carbonate currently available for use,is too large, thereby leading to an substantially ineffective scrubbingprocess.

The dominant process for chemical pulping in the paper industry is thealkaline “Kraft” process which uses sodium hydroxide and sodium sulfideas the primary chemical constituents. In order to make the Kraft pulpingprocess economically feasible, the chemicals are regenerated in a seriesof steps, including: 1) washing the spent chemicals and digested woodsubstance out of the “pulp” and collecting the resultant “weak blackliquor” in large tanks; 2) evaporating the liquor in preparation forburning in a Kraft recovery boiler which produces steam for energyrecovery and molten “smelt” for chemical recovery, wherein the smeltdrops into a tank where it is mixed with water to form “green liquor”which contains sodium carbonate and sodium sulfide; 3) the sodiumcarbonate is reconverted to sodium hydroxide by using calcium oxidewherein the calcium oxide is converted into a finely divided calciumcarbonate called “lime mud” suspended in a regenerated pulping liquor;and 4) the calcium oxide is regenerated by burning the lime mud in alime kiln. Before the lime mud can be burned in a cost-effective way,however, the lime mud must be separated from the regenerated pulpingliquor and washed. After intensive washing and denaturing steps, thelime mud contains primarily calcium carbonate with trace amounts ofcalcium hydroxide and sodium hydroxide. The calcium carbonate in thelime mud ranges in size from less than 1 to greater than 120 microns.

The regeneration of the chemicals in the Kraft pulping system, however,is not entirely effective. Typically, unreactive contaminants come from:(1) The wood used to make the pulp; (2) Corrosion and erosion of pipingand equipment; (3) sulfur compounds from the pulping liquors; and (4)sulfur gasses burned in the rotary lime kiln. A residual level ofcontaminants in the lime mud results in a reburned lime having decreasedcausticizing efficiencies which translates into higher energy coststhroughout the Kraft pulping process. Also, since the lime kiln is oftenthe bottleneck to higher pulp production rates, contaminants in thereburned lime can decrease overall pulp production and concomitantlyincrease energy costs. If the pulp mill also has a bleach cycle, thecontaminants lower the brightness control of the pulp, therebyincreasing the bleaching costs of the pulp production system.

Thus it is an object of the present invention to provide a method ofobtaining a particulate calcium carbonate having an average particlesize less than about 12 microns from a pulp mill.

It is another object of the present invention to provide a method ofoptimizing the operation of the recausticizing cycle in a pulp millthereby reducing the energy costs throughout the recausticizing cycleand maximizing pulp production.

It is a further object of the present invention to provide a method ofapplying a particulate calcium carbonate having an average particle sizeless than about 12 microns to a variety of applications wherein the sizeof the calcium carbonate particles is of concern.

These and other objects of the present invention will become apparent inlight of the present Specification, claims, and Drawings.

SUMMARY OF THE INVENTION

The present invention comprises a method of obtaining particulatecalcium carbonate having an average particle size less than about 12microns. The method comprises the steps of a) withdrawing from a pulpmill a mixture containing calcium carbonate; b) treating the mixture toremove contaminants contained in the mixture to produce a treatedmixture containing calcium carbonate; and c) recovering from the treatedmixture particulate calcium carbonate having an average particle sizeless than about 12 microns.

In a preferred embodiment, the step of withdrawing from a pulp mill amixture containing calcium carbonate further includes that the mixturecontaining calcium carbonate may be withdrawn from either the pulp milllime mud storage tank, the discharge of the mud filter, or the pulp milldust control system; from all of the pulp mill lime mud storage tank,the discharge of the mud filter, and the pulp mill dust control system;or from combinations thereof.

In yet another preferred embodiment, the step of withdrawing from a pulpmill a mixture containing calcium carbonate further includes that themixture containing calcium carbonate is withdrawn from the pulp millrecausticizing cycle at a constant rate so as to require fresh calciumto be added to the recausticizing cycle at a rate greater than about 25percent by weight of the requirements of the recausticizing cycle.

In another embodiment, the step of withdrawing from a pulp mill amixture containing calcium carbonate further includes that the mixturecontaining calcium carbonate is being withdrawn from the pulp millrecausticizing cycle in staggered batches so as to require fresh calciumto be added to the recausticizing cycle at a rate greater than about 25percent by weight of the requirements of the recausticizing cycle.

The present invention also contemplates a method of obtainingparticulate calcium carbonate having an average particle size about 12microns. The method comprises the steps of a) withdrawing from a pulpmill a mixture containing calcium carbonate; b) segregating theparticulate calcium carbonate from the mixture containing calciumcarbonate within the pulp mill prior to withdrawing the particulatecalcium carbonate from the pulp mill; and c) recovering from thesegregated particulate calcium carbonate, a particulate calciumcarbonate having an average particle size less than about 12 microns.

The present invention further contemplates a method of optimizing theoperation of the recausticizing cycle in a pulp mill. The methodcomprises the steps of a) withdrawing from the pulp mill recausticizingcycle a mixture containing particulate calcium carbonate; b) injectingan effective amount of a fresh calcium containing compound selected fromthe group consisting of either calcium oxide or calcium carbonate, intothe recausticizing cycle to replace the withdrawn mixture; c) treatingthe withdrawn mixture to substantially remove contaminants in themixture to produce a treated calcium carbonate mixture; and d)recovering from the treated calcium carbonate mixture a particulatecalcium carbonate having an average particle size less than about 12microns.

The present invention also contemplates a method of adjusting andenhancing the pH of soil. The method comprises the steps of a)withdrawing from a pulp mill a mixture containing calcium carbonate; b)treating the mixture to produce substantially contaminant free treatedmixture containing particulate calcium carbonate having an averageparticle size less than about 12 microns; c) admixing an effectiveamount of water to the treated mixture containing calcium carbonatehaving an average particulate size of less than about 12 microns toprovide a sprayable calcium carbonate slurry; and d) spraying aneffective amount of the sprayable calcium carbonate slurry onto the soilto penetrate the soil to a predetermined depth in order to adjust the pHof the soil. Also, if the soil is low in magnesium, a magnesiumcontaining compound, such as Mg(OH)₂, having a substantially similarparticulate size as of that of the calcium carbonate, may be added tothe slurry as well. If the soil is highly compacted, a penetrant aid mayalso be added to the slurry in order to allow the calcium carbonate topenetrate to the roots. The use of glacier water containing suspendedmicro-nutrients may also be used as a dilutant for the calcium carbonateslurry.

In a preferred embodiment of the invention, the method further comprisesthe steps of a) allowing a predetermined amount of time to elapse topermit the calcium carbonate in the sprayable slurry to penetrate thesoil; and b) measuring the pH of the soil after the predetermined amountof time.

The present invention also comprises a method for reducing acid gascontaminants from furnace and post furnace regions of power boilers,recovery boilers, and other gas streams of such constituents. The methodcomprises the steps of a) withdrawing from a pulp mill a mixturecontaining calcium carbonate; b) treating the mixture to produce asubstantially contaminant free treated mixture containing particulatecalcium carbonate having an average particle size less than about 12microns; and c) injecting an effective amount of the treated mixturecontaining calcium carbonate having an average particulate size of lessthan about 12 microns into a coal stack burning assembly.

The present invention further comprises a method for producing a fillerfor plastics and unbleached pulp or paper. The method comprises thesteps of a) withdrawing from a pulp mill a mixture containing calciumcarbonate; b) treating the mixture to produce substantially contaminantfree treated mixture containing particulate calcium carbonate having anaverage particle size less than about 12 microns; and c) injecting aneffective amount of the treated mixture calcium carbonate having anaverage particulate size of less than about 12 microns into a fiberproducing assembly.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of the pulping andrecausticizing cycles of a Kraft pulping mill.

FIG. 2 is a diagrammatic representation of the recausticizing cycle of aKraft pulping mill of the present invention.

FIG. 3 is a diagrammatic representation of the recausticizing cycle of aKraft pulping mill a second embodiment of the present invention.

FIG. 4 is a diagrammatic representation of the recausticizing cycle of aKraft pulping mill a fifth embodiment of the present invention.

FIG. 5 is a diagrammatic representation of the recausticizing cycle of aKraft pulping mill a fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not limited in its applicationto the details of construction and the arrangements of the componentsset forth in the following description or illustrated in the drawings.The invention is capable of other embodiments or of being practiced orcarried out in various ways. Also, it is to be understood that thephraseology and terminology employed herein is for the purpose ofdescription and should not be regarded as limiting.

The present invention provides a method for obtaining particulatecalcium carbonate having an average particle size less than about 12microns. The method comprises the steps of withdrawing from a pulp milla mixture containing calcium carbonate, treating the mixture to removecontaminants contained in the mixture to produce a treated mixturecontaining calcium carbonate, and recovering from the treated mixturecalcium carbonate having an average particle size less than about 12microns. Thereby, calcium carbonate is produced from a Kraft pulp millrecausticizing cycle. Calcium carbonate from the pulp mill is formed,classified and segregated under controlled conditions which provide forthe calcium carbonate necessary for strict industrial and agriculturalspecifications. The method of the invention produces a calcium carbonatehaving 1) reduced quantities of trace contaminants; 2) smaller overallaverage particle size; and/or 3) a mixture segregated into specific sizeranges.

Pulping and recausticizing cycles 10 of a Kraft pulp mill are showngenerally in FIG. 1. A Kraft pulping process 12 generally includes thecombining of sodium hydroxide and sodium sulfide and organic woodmatter. The sodium hydroxide and sodium sulfide solubilize the lignin inthe organic wood matter thereby releasing the wood fibers which are tiedtogether in the organic wood matter by the lignin. In particular, thesodium binds with the lignin which in turn solubilizes the lignin. Afterthe lignin has been chemically removed from the organic wood matter, thereleased wood fibers are removed from the process and used in otherpaper making processes. The solubilized lignin, excess sodium hydroxideand sodium sulfide, and other trace impurities are washed out of thewood fibers and placed into a recovery boiler. In the recovery boiler,the organic matter is burned off and sodium carbonate and sodium sulfideare produced. The sodium carbonate and sodium sulfide combination isgenerally referred to as “smelt”, and is moved from the Kraft pulpingprocess 12 to a smelt dissolving tank 14 via smelt dissolving tankconduit 15.

In the smelt dissolving tank 14, the smelt is mixed with water to form aslurry commonly referred to as a “Green Liquor.” The Green Liquor isthen moved from the smelt dissolving tank 14 to a Green Liquor clarifier16 via Green Liquor clarifier conduit 19. The Green Liquor contains, inaddition to the sodium carbonate and sodium sulfide, trace amounts ofsolids such as burned dark carbon and metallics. The Green Liquorclarifier 16 separates the liquid (which contains the sodium carbonateand sodium sulfide) from the solids. The Green Liquor clarifier 16settles the solids out of solution and an outlet 17 allows for thecontrolled removal of the solids in such a manner as to minimize theremoval of the liquid. In such a manner, the Green Liquor issubstantially cleaned of solid contaminants.

The clean Green Liquor thereafter leaves the Green Liquor clarifier 16and enters a slaker 18. The slaker 18 introduces an amount of calciumoxide to the Green Liquor (which contains sodium carbonate and sodiumsulfide). The calcium oxide therein reacts with the sodium carbonate toproduce sodium hydroxide and calcium carbonate, while the sodium sulfideremains in solution and does not react. The calcium carbonateprecipitates out of solution and the solution which contains the sodiumhydroxide and sodium sulfide is commonly referred to as a “WhiteLiquor.” After the reaction has occurred, the precipitated calciumcarbonate and the White Liquor are moved to a White Liquor clarifier 20.

The White Liquor clarifier 20 separates the White Liquor solution fromthe precipitated calcium carbonate. The White Liquor solution isconveyed back to the Kraft pulping process 12 via the White Liquorconduit 21. The White Liquor, containing sodium hydroxide and sodiumsulfide, is therein reused to separate the wood fibers from the ligninin the Kraft pulping process 12, as described hereinabove.

The precipitated calcium carbonate (also referred to as “lime mud”) inWhite Liquor clarifier 20 is removed from White Liquor clarifier 20 andconveyed to mud washer 24 via lime mud conduit 22. The lime mud whichexits the White Liquor clarifier 20 is approximately 50% calciumcarbonate suspended in a residual amount of White Liquor. Mud washer 24adds water to the lime mud and White Liquor suspension in order todilute the lime mud further. After the water has been added to the limemud, the lime mud is allowed to settle out of the solution. The lime mudis then removed from the mud washer 24 and conveyed to the mud storage26 via mud storage conduit 28. The remaining diluted White Liquor in mudwasher 24 is pumped into the Kraft pulping process 12 by White Liquorpumping apparatus 30.

The lime mud entering the mud storage 26 is approximately 25-35% solids.Further, the calcium carbonate in the lime mud ranges in size from lessthan 1 micron to greater than 120 microns. The lime mud is thereafterconveyed from mud storage 26 to a mud filter 32 via a mud filter conduit34. The mud filter 32 washes the lime mud with water and separates thelime mud from the resulting solution. After the washing, the lime mud isremoved from the mud filter 32 and transported to a rotary lime kiln 36via rotary lime kiln conduit 38. The lime mud leaving the mud filter 32and entering rotary lime kiln 36 is approximately 70-80% solids.

The rotary lime kiln 36 converts the calcium carbonate in the lime mudto calcium oxide through a calcination process. After the calcinationprocess has finished, the converted calcium oxide is transported to astorage silo 40 by means of a silo conduit 42. The storage silo 40 holdsthe converted calcium oxide until it is needed by the slaker 18 in theconversion of sodium carbonate and calcium oxide to sodium hydroxide andcalcium carbonate. In this manner, the sodium hydroxide and sodiumsulfide used in the Kraft pulping process 12 are repeatedly regeneratedfor subsequent use.

The pulping and recausticizing cycles of the Kraft pulp mill, describedabove, oftentimes accumulate unreactive contaminants, such as 1)contaminants from the particular wood that is used for pulping, 2)unburned contaminants from the Kraft pulping process, 3) iron compoundsfrom mill conduit piping, and 4) various other impurities found in thechemicals introduced into the process in order to make up the chemicallosses which occur during processing. Furthermore, unreactive calciumsulfate may be formed from sulfur dioxide which reacts with the calciumoxide within the rotary lime kiln 36. Contaminant build-up results in 10to 20% more calcium oxide (reburned lime) being needed in the slaker 18for each conversion reaction of sodium carbonate to sodium hydroxide andcalcium carbonate. The contaminants may also fuse the small calciumcarbonate particles produced in the conversion reaction of sodiumcarbonate to sodium hydroxide into larger calcium carbonate agglomerateswhich tend to make them less desirable and/or less reactive.

The present invention, as shown in FIG. 2, contemplates the removal ofthe precipitated calcium carbonate from the recausticizing cycle 23,treating the recovered precipitated calcium carbonate to remove anyresidual contaminants remaining therein, and recovering from the treatedmixture a particulate calcium carbonate having an average particle sizeless than about 12 microns. As described above, the precipitated calciumcarbonate enters White Liquor clarifier 20 which thereby separates theWhite Liquor solution from the precipitated calcium carbonate. Theprecipitated calcium carbonate is then conveyed to mud washer 24 vialime mud conduit 22. After being washed, the lime mud is conveyed to mudstorage 26 via mud storage conduit 28. The precipitated calciumcarbonate and any residual White Liquor solution is thereafter removedfrom mud storage 26 to a storage assembly 46 via a storage assemblyconduit 44. It is also contemplated that any excess soluble hydroxidecan be treated by either (1) oxidizing the mud in the mud storage inorder to stabilize the sulfides and/or (2) using carbonated water towash the mud on the mud filter.

It is also contemplated, and as shown in FIG. 3, that the untreatedcalcium carbonate solution be withdrawn from mud filter 32 to a storageassembly 46 a via a storage assembly conduit 44 a. Furthermore, thecalcium carbonate, as shown in FIG. 4, may be removed from the rotarylime kiln 36 to a storage assembly 46 b via a storage assembly conduit44 b, or from rotary lime kiln dust screens (not shown) or by anycombination thereof. By choosing the withdrawal site of the untreatedcalcium carbonate solution, the calcium carbonate particle sizedistributions can be more exactingly controlled. It is also contemplatedthat calcium carbonate particles having an average size less than about12 microns be removed from the recausticizing cycle 23, as shown in FIG.5, by pumping the solution from the mud storage 26 through ahydrocyclone 48 or other classification device. The material having anaverage size greater than about 12 microns is returned to the mudstorage 26 via a return conduit 50. The material having an average sizeless than about 12 microns is conveyed to the storage assembly 46 viaconduit 52 for further treatment and/or segregation.

The calcium carbonate taken off mud filter 32, which is typically atabout 70-80% solids, may be dried with a small rotary dryer or otherdrying device (not shown) in order to provide a calcium carbonate whichmay be more economically and efficiently shipped. For certainapplications, sufficient dryness could be obtained by adding additionalcalcium oxide or calcium carbonate to the removed and treated calciumcarbonate, followed by drying the combination with a rotary dryer. Itwould be apparent to one of ordinary skill in the art that any methodwhich can efficiently dry the removed calcium carbonate is contemplatedfor use.

Storage assembly 46 is a storage facility capable of treating thecalcium carbonate solution in order to further remove any residualcontaminants which may be in the solution. The calcium carbonatetreatment may consist of single or multiple washings with water or anyother suitable chemical treatment. It is also contemplated that thecalcium carbonate treatment involve a chemical treatment, whereby anyimpurities or contaminants in the mixture are solubilized and removedand/or the impurities or contaminants may be converted to some harmlessor beneficial by-product. It is also contemplated that two or morediluting and/or rethickening steps may be required in order to removethe impurities or contaminants. Also, if the calcium carbonate solutioncontains excess sodium, the storage assembly 46 may also contain aconductivity meter capable of measuring the amount of sodiumcontaminants in the solution and maximize the purification treatment ofthe calcium carbonate solution to remove the contaminant sodium. Inparticular, the conductivity of the calcium carbonate solution should beless than 1800 micromhos. In any event, one of ordinary skill in the artwould understand that any method of treating the precipitated calciumcarbonate solution, so as to substantially remove impurities ofcontaminants remaining therein, is contemplated for use.

The storage assembly 46 is also capable of recovering, from theabove-described treated calcium carbonate, a particulate calciumcarbonate having an average particle size less than about 12 microns. Inone embodiment, the storage assembly 46 may contain a hydrocyclone, orother such classification device which would be known to one of ordinaryskill in the art. The hydrocyclone, or other segregation device, iscapable of removing a particular range of particle sizes based upon theunit design and the resultant pressure drop across the unit. In terms ofoperational control, a higher pressure drop shifts the particle sizesplit so that the discharge from the top of the cyclone contains lesstotal mass flow of the particulate calcium carbonate, but will containparticulate calcium carbonate having a smaller particle size. It is alsocontemplated that the recovery of particulate calcium carbonate havingan average size less than about 12 microns may involve more than onesegregation step. For instance, two or more passes through thehydrocyclone or other segregation device may be required in order toobtain a calcium carbonate particulate having an average size less than12 microns.

Alternate methods may be employed within the storage assembly 46 inorder to segregate out a particulate calcium carbonate having an averageparticle size less than 12 microns. A settling chamber or clarifier maybe included with the storage assembly 46 which allows the largerparticles to settle out of solution first, thereby allowing for thepreferential selection of smaller particles. Also, a liquid grinding orimpacting assembly may be included in the storage assembly 46 in orderto break up larger sized particles that may escape removal by thehydrocyclone or other segregating device. Of course, any of thesesegregating devices may be used singularly or in combination with any ofthe others and may also comprise an additional assembly separate from,yet connected to, storage assembly 46. In any event, one of ordinaryskill in the art will understand that any method of segregating acalcium carbonate particulate having an average particle size less than12 microns out of solution, is contemplated for use.

Pulp mills traditionally operate the recausticizing cycle so that up to1.5% sodium, expressed as Na₂O, is in the process since this amount ofsodium helps to reduce dust and encourages the formation of a pelletsized calcium oxide product. This minute level of sodium, however,causes the calcium carbonate to form agglomerates of an increased size,as described hereinabove. In this regard, the effect of the sodium onthe calcium carbonate appears to be cumulative. In order to minimize thenumber of calcium carbonate agglomerates formed, the calcium carbonateis preferentially removed from the mud storage 26 before it has beenthrough rotary lime kiln 36.

Currently, pulp mills add approximately 5% calcium oxide to therecausticizing cycle (hereinafter referred to as “new lime”) in order topurge an amount of the contaminants formed during each cycle. Even withthis purge in the recausticizing cycle, however, the amount ofcontaminants in the system is such that the contamination levels in thecycle are increased by a factor of twenty. The effectiveness of anycontaminant purge is expressed by the “percentage available calciumoxide” found in the material leaving rotary lime kiln 36. For example,calcium oxide produced in the rotary lime kiln 36 often has less thaneighty-five percent (85%) calcium oxide available, whereas new lime hasgreater than ninety-two percent (92%) calcium oxide available.Therefore, the more calcium oxide produced in the rotary lime kiln 36that is used for each ton of pulp produced, causes several dead loadcosts. Furthermore, if the rotary lime kiln 36 is the bottleneck in therecausticizing cycle 23, there is a correlative loss in pulp productionin the sodium cycle 13.

It is contemplated that the removal of the calcium carbonate from therecausticizing cycle 23 be accomplished in such a manner so that newcalcium, such as calcium oxide, be added to the recausticizing cycle 23at a rate greater than about 25 percent by weight of the requirements ofthe recausticizing cycle 23. The mixture containing the calciumcarbonate may be withdrawn from the recausticizing cycle 23 in either acontinuous stream or in staged increments, so long as the 25 percent byweight withdrawal requirement is met.

The present invention also contemplates the optimization of a Kraft pulpmill. The method of optimizing the Kraft pulp mill includes the stepsof: (A) withdrawing from the pulp mill recausticizing cycle a mixturecontaining particulate calcium carbonate; (B) injecting an effectiveamount of a fresh calcium containing compound selected from the groupconsisting of calcium oxide and calcium carbonate, into therecausticizing cycle to replace the withdrawn mixture; (C) treating thewithdrawn mixture to substantially remove contaminants in the mixture toproduce a treated calcium carbonate mixture; and (D) recovering from thetreated calcium carbonate mixture a particulate calcium carbonate havingan average particle size less than about 12 microns.

It is contemplated that the calcium carbonate be withdrawn at a rategreater than 5% and that the new lime be introduced into storage silo 40thereby substantially purging the recausticizing cycle 23 ofcontaminants, and eliminating the rotary lime kiln 36 as a pulp millpulp production bottleneck. Traditionally, the limiting factor on thedegree of the purge was based on economic considerations since thepurged “lime mud” or calcium carbonate mixture had to be hauled away orstored in some manner. If the rotary lime kiln 36 is the bottleneck tohigher pulp production rates, reducing the amount of calcium oxideneeded to be produced by the rotary lime kiln 36 results in the calciumoxide being added to the slaker having a higher percent availablecalcium oxide content which thereby translates into increased pulpproduction capacity without a significant capital expenditure. Theoptimization of the recausticizing cycle 23 of the pulp mill translatesinto lower overall energy costs within Kraft pulping process 12. It isalso contemplated that any residual moisture in the calcium carbonatemay be reduced and/or eliminated by passing it through an additionalrotary dryer or by using the existing rotary lime kiln as a dryer ifproduction requirements are not affected.

If calcium carbonate is removed from the rotary lime kiln 36 or dustscreens, calcium carbonate dust levels within the rotary lime kiln 36will be minimized. Kraft pulp mills that utilize bleach plants willbenefit from better brightness control and lower bleaching costs due tothe decrease in pulp mill contaminants that are removed with the calciumcarbonate and the increased reactivity of the calcium oxide that isintroduced into slaker 18.

Furthermore, the rotary lime kiln 36 is the back end of the pulpingprocess and there is a general lack of knowledge in the field as to therecausticizing cycle. For example, pulp mills generally do not have adifferent set of specifications and procedures for the addition of freshcalcium oxide. In fact, operators often attribute poor settling of thecalcium oxide mud with the addition of fresh calcium oxide in general,rather than the fact that there may be too high a concentration ofcalcium oxide in the recausticizing cycle overall. It is contemplatedthat fresh calcium oxide, therefore, be added at a rate of 10-20% lessthan the amount of calcium oxide produced by the rotary lime kiln 36.The calcium oxide mud produced by the addition of 10-20% less freshcalcium oxide versus calcium oxide produced in the rotary lime kiln 36settles very quickly even though the particle size is much smaller.

It is contemplated that the calcium carbonate produced by the presentinvention will also benefit users of calcium carbonate products bylowering the capital costs inherent in traditional methods of processingcalcium carbonate, such as manual grinding or crushing systems. Ratherthan expensive rock handling and grinding equipment, the optimization ofpulp mill operations and the resultant production of calcium carbonaterequires minimal (1) wet or dry product storage; (2) transfer andtransport equipment; and (3) classification or segregation equipment.The present invention, therefore, contemplates a method of optimizing apulp mill recausticizing cycle and in return producing a particulatecalcium carbonate product which is highly reactive and economicallyproduced.

The present invention also contemplates a method for adjusting andenhancing the pH of the soil. The method generally entails the use ofthe calcium carbonate particles removed from a Kraft pulp mill. Inparticular, the method comprises the steps of (A) withdrawing from apulp mill a mixture containing calcium carbonate; (B) treating themixture to produce a substantially contaminant free treated mixturecontaining particulate calcium carbonate having an average particle sizeless than about 12 microns; (C) admixing an effective amount of water tothe treated mixture containing calcium carbonate having an averageparticulate size of less than about 12 microns in order to provide asprayable calcium carbonate slurry; and (D) spraying an effective amountof the sprayable calcium carbonate slurry onto the soil in order topenetrate the soil to a predetermined depth in order to adjust the pH ofthe soil. The method may also comprise the additional steps of (1)allowing a predetermined amount of time to elapse in order to permit thecalcium carbonate in the sprayable slurry to penetrate the soil; and (2)measuring the pH of the soil after the predetermined amount of time. Inthis manner the soil may be treated so as to neutralize its acidicnature through a method which is quick, efficient, and easilyadministered.

Particle size for agricultural calcium carbonate traditionally averages84 microns or more. With an average particle size of less than 12microns, the full benefit of the calcium carbonate treatment will berecognized in the same growing season as its application. The smallparticle size of the calcium carbonate removed from the pulp mills,which is less than about 12 microns, is well below the particle size ofground calcium carbonate which is currently produced and used by theagriculture industry. In fact, attempting to grind limestone to aparticle size of less than about 12 microns would be economicallyinfeasible. Furthermore, the relatively small size of the removedcalcium carbonate results in a product having a high volume to surfacearea ratio. Thus, the removed and treated calcium carbonate is highlyreactive and results in a relatively inexpensive and effective soiltreatment.

It is also contemplated that the removed and treated calcium carbonatecan further be improved by grinding the particles into a desired sizerange even less than 12 microns through the use of a touch-up grinder.For instance, the power industry often uses calcium carbonate to scrubsulfur dioxide from their flue gases which are exhausted into theatmosphere. In order to maximize the calcium carbonate's reaction andapplication efficiency, it is ground to its smallest economic particlesize.

The present invention also contemplates a method for reducing acid gascontaminants from furnace and post furnace regions of power boilers,recovery boilers, and other gas streams of such constituents. The methodcomprises the steps of (1) withdrawing from a pulp mill a mixturecontaining calcium carbonate; (2) treating the mixture to produce asubstantially contaminant free treated mixture containing particulatecalcium carbonate having an average particle size less than about 12microns; and (3) injecting an effective amount of the treated mixturecontaining calcium carbonate having an average particulate size of lessthan about 12 microns into a coal stack burning assembly. The calciumcarbonate injected into the coal stack burning assembly is highlyreactive with the acid gases, such as SO₂, HCl, because of its highsurface area to volume ratio. The calcium carbonate reacts with the acidgases to produce calcium sulfite, calcium sulfate and calcium chlorideand H₂O, thereby lowering the amount of acid gases emitted from the coalstack burning assembly. In such a manner, the amount of acid gascontaminants is substantially reduced.

The present invention further contemplates a method for producing afiller for unbleached pulp or paper. The method comprises the steps of(1) withdrawing from a pulp mill a mixture containing calcium carbonate;(2) treating the mixture to produce a substantially contaminant freetreated mixture containing particulate calcium carbonate having anaverage particle size less than about 12 microns; and (3) injecting aneffective amount of the treated mixture calcium carbonate having anaverage particulate size of less than about 12 microns into a fiberproducing assembly.

Because of its lower brightness, the calcium carbonate produced by theinvention would have to be bleached in order that it be used in theproduction of paper from pulp. The brightness of the particulate calciumcarbonate can also be enhanced by the addition of bleaching agents atthe lime mud filter. However, it may be used in unbleached paper andpaper board, and its use in such a product would cut pulp costs.However, even if the produced calcium carbonate is too dark to put intounbleached linerboard or sack paper, it may be treated with a bleachingagent and thereby improve its brightness.

The calcium carbonate produced by the invention could also be readilyused as a substitute for ground calcium carbonate which is used in themanufacture of some types of paints which do not require a normalbrightness factor, such as for use in dark colors of paint. It is alsocontemplated that the calcium carbonate could be bleached to improve thefinal brightness of the paint. As with paint, the addition of thecalcium carbonate to plastics is also contemplated after bringing theproduct to dryness.

Thus, it should be apparent that there has been provided in accordancewith the present invention a method for obtaining particulate calciumcarbonate having an average particle size less than about 12 micronsthat fully satisfies the objectives and advantages set forth above.Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace such alternatives, modificationsand variations that fall within the spirit and broad scope of theappended claims.

What we claim is:
 1. A method for obtaining particulate calciumcarbonate product having an average particle size less than 12 microns,comprising the steps of: adding calcium to a recausticizing cycle of akraft pulp mill at a rate greater than 25 percent by weight of therequirements of the recausticizing cycle; withdrawing from the kraftpulp mill an alkaline mixture containing calcium carbonate wherein thealkaline mixture containing calcium carbonate is withdrawn during therecausticizing cycle; treating the alkaline mixture containing calciumcarbonate so as to remove contaminants contained in the alkaline mixtureto thereby produce a treated alkaline mixture substantially free ofcontaminants and containing calcium carbonate; and recovering from thetreated alkaline mixture a particulate calcium carbonate product havingan average particle size less than 12 microns, wherein the treatedalkaline mixture has been treated such that the particulate calciumcarbonate product has a substantially neutral pH.
 2. The method of claim1, wherein the mixture containing calcium carbonate is withdrawn from alime mud storage tank.
 3. The method of claim 1, wherein the mixturecontaining calcium carbonate is withdrawn from a discharge of the mudfilter.
 4. The method of claim 1, wherein the mixture containing calciumcarbonate is withdrawn from a pulp mill dust control system.
 5. Themethod of claim 1, wherein the mixture containing calcium carbonate iswithdrawn from a plurality of a lime mud storage tanks, a mud filter, apulp mill dust control system, and combinations thereof.
 6. The methodof claim 1, wherein the calcium added is selected from the groupconsisting of calcium oxide, calcium carbonate and combinations thereof.7. The method of claim 6, wherein the calcium added has an averageparticle size greater than 12 microns.
 8. The method of claim 1, whereinthe contaminants moved from the alkaline mixture containing calciumcarbonate are selected from the group consisting of sodium compounds,sulfide compounds and combinations thereof.